The invention relates to a gas-measuring system having a measurement sensor which records the exhaust gas from an internal combustion engine and has an outer electrode, which is exposed to the exhaust gas to be measured, a first measuring cell, a second measuring cell, which is connected to the first measuring cell and in which a measuring electrode is arranged, and a reference electrode, which is exposed to the ambient air, the measuring cells being located in a solid electrolyte, and all the electrodes being in contact with the solid electrolyte, a circuit, which in a first pump flow pumps oxygen ions between the first measuring cell and the outer electrode, in a second pump flow pumps oxygen ions between the second measuring cell and the outer electrode and in a third pump flow pumps oxygen ions between the measuring electrode and the outer electrode, and which comprises a control unit, which records Nernst voltages between the measuring cells and a reference electrode and controls the second pump flow to a desired value by evaluating the recorded Nernst voltages. The invention also relates to a method for operating a measurement sensor that records an exhaust gas from an internal combustion engine and has an outer electrode, which is exposed to the exhaust gas to be measured, a first measuring cell, a second measuring cell, which is connected to the first measuring cell and in which a measuring electrode is arranged, and a reference electrode, which is exposed to the ambient air, the measuring cells being located in a solid electrolyte and all the electrodes being in contact with the solid electrolyte, and for operating a circuit, which pumps oxygen ions in a first pump flow between the first measuring cell and the outer electrode, pumps oxygen ions in a second pump flow between the second measuring cell and the outer electrode, and pumps oxygen ions in a third pump flow between the measuring electrode and the outer electrode, Nernst voltages being recorded between the measuring cells and a reference electrode and the second pump flow being controlled to a desired value by evaluating the recorded Nernst voltages.
Systems of this type, with measurement sensor and circuit, are used as standard for measuring the NOx concentration in the exhaust gas from an internal combustion engine, as described for example in EP 0 816 836 A2. The measurement sensor of the system has two measuring cells in a body made from zirconium oxide which conducts oxygen ions. At this sensor, the circuit implements the following measurement concept: in the first measuring cell, which is supplied with the measurement gas via a diffusion barrier, a first oxygen concentration is set by means of a first oxygen ion pump flow, the intention being for there to be as far as possible no decomposition of NOx. In the second measuring cell, which is connected to the first via a further diffusion barrier, the oxygen content is reduced further by means of a second oxygen ion pump flow. To set the oxygen ion pump flows, the circuit taps off a Nernst voltage in the respective measuring cells, reference always being made to an oxygen content to which a reference electrode is exposed, usually that of the ambient air. Furthermore, the second oxygen ion pump flow and the Nernst voltage of the measuring cell are controlled to a desired value.
The decomposition of NOx at a measuring electrode located in the second measuring cell leads to a third oxygen ion pump flow, which is a measure of the NOx concentration. The entire measurement sensor is in this case brought to an elevated temperature, e.g. 750° C., by means of an electrical heater.
The NOx measuring system mentioned is able to record oxides in the exhaust gas. However, it is not sufficiently suitable for other types of gas components or gas parameters. For components of this type, it has hitherto been imperative to use separate sensors. This applies in particular to the field of automotive engineering, where it is known to use separate oxygen or lambda sensors for oxygen measurement in addition to the above-mentioned NOx measuring system. The outlay which is involved in using a plurality of sensors is of course undesirable.